As shown in our copending application Ser. No. 250,804, aromatic bis(ether phthalic anhydride) of the formula, ##STR1## can be made by effecting reaction between an aqueous mixture of phthalic acid and a triorganoamine imide-anhydride exchange catalyst with molten aromatic bis(ether phthalimide) of the formula, ##STR2## followed by the extraction of the resulting imide-anhydride exchange mixture with an inert organic solvent, where R is a C.sub.(6-30) divalent aromatic organic radical, where R.sup.1 is a monovalent organo radical selected from the class consisting of C.sub.(1-8) alkyl radicals, and C.sub.(6-13) aromatic radicals.
It is further taught in Ser. No. 250,804 that the extraction of the imide-anhydride exchange mixture is effected at a temperature of about 200.degree. C. and a pressure of about 500 psi. In order to ensure the recovery of an exchange product having at least 97 mole percent of aromatic bis(ether phthalic anhydride), it has been found that the imide-anhydride exchange mixture has to be extracted at least three times with organic solvent. As a result, high pressure equipment had to be utilized when operating at temperatures of about 200.degree. C., particularly where the organic solvent was a volatile material, such as toluene. As a result, the overall process for making the aromatic bis(ether phthalic anhydride) of formula (1) using the method of Ser. No. 250,804 had to be carefully monitored to satisfy safety regulations.
The present invention is based on the discovery that aromatic bis(ether phthalic anhydride) of formula (1) can be made from molten aromatic bis(ether-N-organophthalimide) of formula (2) in accordance with the aforementioned imide-anhydride exchange reaction, where imide-anhydride exchange product can be recovered having at least 97 mole percent of aromatic bis(ether phthalic anhydride) by increasing the number of times the imide-anhydride exchange mixture is extracted with the inert organic solvent. For example, if the extraction of an imide-anhydride exchange mixture, which has reached steady-state requires three extractions at 200.degree. C. and 500 psi, an equivalent level of extraction efficiency can be obtained using six passes at about 130.degree.-140.degree. C.